Re: [PATCH] sched: Fix NULL user_cpus_ptr check in dup_user_cpus_ptr()

[Waiman Long]

On 11/23/22 06:20, Peter Zijlstra wrote:
> On Tue, Nov 22, 2022 at 02:06:53PM -0500, Waiman Long wrote:
> > In general, a non-null user_cpus_ptr will remain set until the task dies.
> > A possible exception to this is the fact that do_set_cpus_allowed()
> > will clear a non-null user_cpus_ptr. To allow this possible racing
> > condition, we need to check for NULL user_cpus_ptr under the pi_lock
> > before duping the user mask.
> >
> > Fixes: 851a723e45d1 ("sched: Always clear user_cpus_ptr in do_set_cpus_allowed()")
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > ---
> >   kernel/sched/core.c | 23 +++++++++++++++++++++--
> >   1 file changed, 21 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 8df51b08bb38..f447a6285ea2 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -2625,7 +2625,14 @@ int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
> >   		      int node)
> >   {
> >   	unsigned long flags;
> > +	cpumask_t *user_mask = NULL;
> The inverse xmas tree is sad :-(
Right. The inverse xmas tree rule. Will fix that.
> >   
> > +	/*
> > +	 * If there is a concurrent sched_setaffinity(), we may miss the
> > +	 * newly updated user_cpus_ptr. However, a non-NULL user_cpus_ptr
> > +	 * is relatively unlikely and it is not worth the extra overhead
> > +	 * of taking the pi_lock on every fork/clone.
> > +	 */
> I think the correct argument is saying the thing is racy and loosing the
> race is a valid situation. IOW, this is the same as if the concurrent
> sched_setaffinity() happens after fork().
Good point, will update the comment.
> >   	if (!src->user_cpus_ptr)
> >   		return 0;
> >   
> > @@ -2633,10 +2640,22 @@ int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
> >   	if (!dst->user_cpus_ptr)
> >   		return -ENOMEM;
> >   
> > -	/* Use pi_lock to protect content of user_cpus_ptr */
> > +	/*
> > +	 * Use pi_lock to protect content of user_cpus_ptr
> > +	 *
> > +	 * Though unlikely, user_cpus_ptr can be reset to NULL by a concurrent
> > +	 * do_set_cpus_allowed().
> > +	 */
> >   	raw_spin_lock_irqsave(&src->pi_lock, flags);
> > +	if (src->user_cpus_ptr)
> > +		cpumask_copy(dst->user_cpus_ptr, src->user_cpus_ptr);
> > +	else
> > +		swap(dst->user_cpus_ptr, user_mask);
> Uhhhh, did you mean to write:
>
> 	if (src->user_cpus_ptr) {
> 		cpumask_copy(user_mask, src->user_cpus_ptr);
> 		swap(dst->user_cpus_ptr, user_mask);
> 	}
>
> ?

Not really. The point is that dst->user_cpus_ptr has been allocated. If 
src->user_cpus_ptr turns out to be NULL, we need to clear 
dst->user_cpus_ptr which is what the swap() does and then free that 
memory after unlock. Will add a comment to make this point clear.

Cheers,
Longman